Three-dimensional (3D) printing includes processes for making a 3D structure based on a digital model. Some types of 3D printing processes build the structure by adding (or “printing”) relatively thin layers of material, one on top of the other, until the desired 3D structure is formed. Other types of 3D printing processes build the structure by adding material three-dimensionally. That is, rather than forming a 3D structure by accumulating layers of material, these processes build the 3D structure by moving a print head in three dimensions, and adding material wherever is needed in order to build the 3D structure.